Do you mean gas-ram cylinder 'air' guns? I've had my Theoben [the original gas-ram powered airgun] since 1988, and the last time I measured it for a off-hand competition [early last month] it was producing exactly the same m/e and velocity as it said it did on the test card that came with it. I hasten to add that -
a. My BIL once left it cocked for almost a year while I was in Japan.
b. It gets used maybe twice a week to keep my hand in - say forty/fifty shots a time.
I still have my springer that my dad bought in a sunday market when I was about eight or nine - I'm now 66.
Both of us look pretty beat up by now, but we still work just fine.
tac